Evaluation of influenza vaccine effectiveness among older adults in Jiaxing, China, 2022–2025: A test negative design-based study

Influenza imposes a substantial burden on older adults, underscoring the need for annual evaluation of vaccine effectiveness (VE). Using a test-negative case-control design, this study assessed VE among patients aged ≥60 y with acute respiratory symptoms in Jiaxing, China, across three influenza seasons (2022-2025). Among 3,214 participants, vaccination provided moderate overall protection against laboratory-confirmed influenza, with adjusted VE of 42.5% (95% CI: 31.2-51.9). Protection was highest in the 2024-2025 season (47.2%) and against influenza A(H1N1) (42.4%). Notably, subgroup analyses revealed significantly higher VE among females, individuals without underlying disease (49.2%), those presenting with high fever (>39°C, 44.3%), and within 90 d post-vaccination (52.3%). These findings affirm the moderate protective effect of influenza vaccines in the older individuals and strongly support the critical importance of timely annual vaccination ahead of the influenza season to mitigate risk in this vulnerable population.